In the practical or successful operation of a ring laser type gyro, it is necessary to adjust the length of the laser optical path to achieve that optimum laser frequency associated with maximum laser gain. Also, adjustment of the alignment of the laser optics is required for stabilizing system losses. In other words, an optimum path length adjustment is sought for maximum laser intensity, while maintenance of an optimum optical path alignment is sought in order to maintain optical cavity losses preferrably at a minimum or at least at a constant value. Thus, by means of both adjustments, a peak laser output intensity is obtained and gyro bias stabilization is improved.
As discussed in co-pending application Ser. No. 768,511 for Laser Optics Control for Ring Laser Gyro, filed February 14, 1977 by Sidney G. Shutt, assignor to Rockwell International Corporation, assignee of the subject application, prior methods for effecting such adjustments have relied upon movable mirrors for path length adjustment and stable geometry of mechanically positioned optical elements. However, obvious limits are imposed with regard to the mechanical tolerances and geometric stability achievable for a given design and choice of materials. Also, such limits are approached only at high unit costs and manufacturing expense. Further, the error sources associated with such limits are subject to change due to thermal expansion, material "creep" and the like.
The above-noted co-pending application Ser. No. 768,511 describes a technique for laser gyro path length and alignment control adjustment of a movable mirror interposed in the laser optical path. In such arrangement, two frequency-coded signals are employed to drive a duo-mode bimorph device whereby the path length is modulated to produce a modulated beam intensity which may be photoelectrically sensed and synchronously demodulated to provide an error path signal for closed loop duo-mode control of the movable mirror. However, such duo-mode control mechanization requires a substantial duplication of electronics for such duo-mode control, as to reduce system reliability, increase system initial acquisition and maintenance costs. For example, the use of two frequency-coded reference signals requires two reference signal generators, while the use of separate control amplifiers involves duplication of the control electronics with consequent design problems of gain and phase tracking or calibration.